1. Field of the Invention
The present invention relates to an image projector which is optimum for application to a projector projecting a color image on a screen etc., more particularly relates to an image projector using a color wheel to split white light emitted from a light source into effective light beams of the R (red), G (green), B (blue), and other wavelength bands shifted by time, modulating the split R, G, and B effective light beams by a liquid crystal panel, digital micro-mirror device (DMD), or other spatial light modulator, and projecting them on a screen etc.
2. Description of the Related Art
Conventionally, this kind of image projector may be roughly divided into two systems. The first system of such an image projector uses three R (red light), G (green light), and B (blue light) liquid crystal panels, DMDS, or other spatial light modulators. It passes the white light emitted from a light source through three R, G, and B diohromic mirrors to split it into R, G, and B effective light beams which it then guides to three spatial light modulators, drives the three spatial light modulators, combines the three R, G, and B optical images obtained by modulation by the three spatial light modulators by a prism system, and projects the result on a screen etc.
Next, as shown in FIG. 21, the second system of such an image projector, shown by reference numeral 1, comprises a white light source 2 comprised by a discharge lamp 2a, a reflection mirror (reflector) 2b, etc. and a condenser lens 3, a transmission type color wheel 4, and a single spatial light modulator 5 arranged in series on an horizontal optical axis P of white light L1 emitted from the light source 2. As shown in FIG. 22A, the transmission type color wheel 4 has formed around the periphery of its wheel surface 4a red filter RF, a green filter GF, and a blue filter BF comprised by R, G, and B color filters 4b dividing the outer periphery into three sections. Note that, as shown in FIG. 22B, there is also a transmission type color wheel 4 with a white filter WF provided in addition to the color filters 4b to thereby divide the outer periphery into four and improve the luminance.
Returning to FIG. 21, the transmission type color wheel 4 is arranged perpendicular to the optical axis P of the white light L1 emitted from the white light source 2. The transmission type color wheel 4 is rotated at a high speed (10 s to 100 s of revolutions per second) by a driving means (not shown). The white light L1 emitted from the white light source 2 strikes rotating positions of the red filter RF, green filter GF, and blue filter BF of the color filters 4b of the transmission type color wheel 4 perpendicularly and passes through the red filter RF, green filter GF, and blue filter BF. As a result, the white light L1 is split into R, G, B, and other effective light beams L2 shifted by time by the transmission type color wheel 4. The effective light beams L2 strike the spatial light modulator 5 at different times. The spatial light modulator 5 modulates the effective light beams L2 to three R, G, and B(W) optical images L3 (and white light for improving the luminance) shifted by time by applying R, G, and B image signals (and a signal for improving the luminance) in synchronization with the time the three or four R, G, and B(W) effective light beams L2 strike it and emit those optical images L3. At this time, the three R, G, and B optical images L3 (and white light for improving the luminance) can be viewed by the human eye superimposed, so the optical images L3 are projected by a projection lens 6 on a screen 7 etc. to thereby project a full color image.
Summarizing the problems to be solved by the invention, the first system of image projector using three spatial light modulators and dichromic mirrors suffers from the problem of the large number of parts and a difficulty in space saving, so is not suitable for compact projectors.
The second system of image projector 1 using the transmission type color wheel 4 and the single spatial light modulator 5 as shown in FIG. 21 has the problem that it is difficult to reduce the height of the overall apparatus.
That is, as shown in FIG. 23, when using the transmission type color wheel 4, the cross-sectional area of the spot SL of the white light L1 striking the transmission type color wheel 4 must be made sufficiently small with respect to the diameter of the transmission type color wheel 4. At the instant the spot of light SL passes a dividing point RGP, GBP, or BRP of the red filter RF, green filter GF, and blue filter BF, two colors of effective light end up to be mixed. If the spatial light modulator 5 operates at the instant a mixed effective light beam L2 strikes it, an optical image L3 having two mixed colors is projected on the screen 7.
To prevent this, it is necessary to turn off the spatial light modulator 5 at the times the spot of light SL passes the dividing points RGP, GBP, and BRP of the red filter RF, green filter GF, and blue filter BF to prevent projection of an optical image L3 of two mixed colors on the screen 7.
FIG. 24 shows the state in which the spatial light modulator 5 is supplied with R, G, and B image signals SR, SG, and SB in synchronization with the striking of the three R, G, and B effective light beams L2. The mixture of two colors of light in an optical image L3 is prevented by setting times T for turning off the spatial light modulator 5 between times of application of the R, G, and B image signal SR, SG, SB.
The off time T of the spatial light modulator 5, however, lowers the luminance of the optical image projected on the screen 7 and causes a lowering of the image quality of the projected full color image.
Accordingly, it is desirable that the spot SL of the emitted light L1 striking the transmission type color wheel 4 be as small as possible. The white light emitted from the discharge lamp 2a or other white light source is originally emitted from a light source with great limitations, however. Even if condensing this white light L1, the white light L1 cannot be focused completely to a single point. Consequently, the spot of light SL has a certain size.
For the reasons described above, as shown in FIG. 25, the diameter of the transmission type color wheel 4 must be made relatively large (normally the diameter-D is not less than approximately 10 cm). In a conventional image projector 1 of the system where in the transmission type color wheel 4 is arranged perpendicular to the optical axis P of the white light L1 emitted from the white source 1, the overall height H of the image projector 1 increases in proportion to the diameter D of the transmission type color wheel 4. Further, If the overall height of H of the image projector 1 increases, the overall volume of the image projector 1 increases, consequently the image projector 1 becomes large in size and heavy in weight. Further, costs are increased due to the increase of the casing material of the image projector 1 etc.
Accordingly, as shown in FIG. 26, it has been considered to make the transmission type color wheel 4 tilt slightly with respect to the optical axis P in order to make the overall height H of the image projector 1 shown in FIG. 25 as small as possible.
When tilting the transmission type color wheel 4 with respect to the optical axis P, however, as shown in FIG. 27A, the spot SL of the white light becomes an ellipse and increases in cross-sectional area. As described above, the time during which the spot of light SL passes the dividing points RGP, GBP, and BRP of the red filter RF, green filter GF, and blue filter BF of the transmission type color wheel 4 becomes longer, the time T for turning off the spatial light modulator 5 shown in FIG. 24 becomes longer, and consequently the luminance of the optical images L3 projected on the screen 7 ends up being lowered.
Note that in order to prevent this, as shown in FIG. 27B, by arranging the position at which the spot of light SL strikes the color wheel 4 directly below or directly above a center of rotation of the transmission type color wheel 4 and adjusting a direction of a short axis of the spot of light SL having an ellipse shape to a rotational direction of the transmission type color wheel 4, the time during which the spot of light SL passes the dividing points RGP, GBP, and BRP will not become that long.
In order to arrange the position where the spot of light SL strikes the color wheel 4 to be directly below or directly above the center of rotation of the transmission type color wheel 4, however, the transmission type color wheel 4 has to be offset above or below to the optical axis P of the white light L1 emitted from the white light source 1. If offsetting the transmission type color wheel 4 above or below the optical axis, however, the overall height H of the image projector 1 ends up increasing.
On the other hand, the three or four color filters 4b constituting the red filter RF, green filter GF, and blue filter BF (or white filter WF) are each designed to transmit a designated color and absorb or reflect light of other colors, that is, unwanted light. That is, the red filter RF transmits only R and absorbs or reflects the unwanted G and B(W) light. The green filter GF transmits only G and absorbs or reflects the unwanted R and B(W) light. The blue filter BF transmits only B and absorbs or reflects the unwanted R and G(W) light.
Accordingly, in a high luminance image projector 1 raising the optical energy of the white light L1 emitted from the white light source 2 so as to project optical images L3 of high luminance, the amount of absorption of the unwanted light at the color filters 4b of the transmission type color wheel 4 increases and the color filter 4b is easily damaged by the heat. Accordingly, it is desirable that the color filters 4b reflect the unwanted light as much as possible.
There is a method of using dichroic mirrors comprising a plurality of layers of coated thin films as the three or four color filters 4b constituting the red filter RF, green filter GF, and blue filter BF (white filter WF) of the transmission type color wheel 4. By using dichroic mirrors, it is possible to make the red filter RF, green filter GF, and blue filter BF (white filter WF) transmit respectively only specific R, G, and B(W) effective light beams L2 and reflect other unwanted light, consequently the amount of absorption of the unwanted light at the color filters 4b decreases and the color filters 4b become resistant to heating and damage.
If using dichroic mirrors as the color filters 4b of the transmission type color wheel 4 shown in FIG. 21, however, when the three or four color filters 4b of the transmission type color wheel 4 are struck by the white light L1 emitted from the white light source 2 and pass the R, G, and B (W) effective light beams L2, as shown by the dotted lines in FIG. 21, the unwanted light L4 reflected by the color filters 4b ends up directly returned to the white light source 1 side. That unwanted light L4 conversely passes through the condenser lens 3 toward the white light source 2 side and is condensed by the reflection mirror 2b to strike and heat the discharge lamp 2a. 
In general, high pressure gas is sealed in the discharge lamp 2a. If the discharge lamp 2a is heated by the unwanted light 14, the high pressure expands and therefore the discharge lamp 2a is easily destroyed. Furthermore, the ventilation around the discharge lamp 2a is very poor since the discharge lamp 2a is surrounded by the reflection mirror 2b. The discharge lamp 2a is therefore located in an environment where the temperature easily rises. Consequently, the discharge lamp 2a easily rises in temperature and is destroyed due to the unwanted light L4.
In order to prevent the heating and destruction of the discharge lamp 2a by the unwanted light L3, the discharge lamp 2a has to be cooled. However, it is extremely difficult to cool the discharge lamp 2a surrounded by the reflection mirror 2b effectively. If using an air cooling fan etc. for forced cooling, a large cooling fan becomes needed. This not only causes the image projector 1 to become larger and heavier, but also increases the power consumption due to the need to drive the large air cooling fan and creates the new problem of the noise caused by the operation of the large air cooling fan.
An object of the present invention is to provide an image projector capable of enlarging the diameter of the color wheel without increasing the overall height of the apparatus.
Another object of the present invention is to provide an image projector capable of preventing heating and damage of the color wheel caused by the absorption of light by the color filters, achieving superior heat dissipation, and saving power.
An image projector of the present invention uses a reflection type color wheel designed to receive white light emitted from a light source into light beams (effective light beams) of different wavelength bands at different times and guiding the light beams to the spatial light modulator without returning them to the light source.
Since an image projector of the present invention comprised as described above uses a reflection type color wheel to receive white light emitted from a light source to light beams of different wavelength bands at different times and directs the light beams to the spatial light modulator without returning them to the light source, it becomes possible to freely set the reflection type color wheel to any desired angle other than a right angle with respect to an optical axis of the white light emitted from the light source and to make the diameter of the reflection type color wheel larger without increasing the overall height of the apparatus.
Alternatively, the image projector of the present invention comprises a reflection type color wheel designed to receive white light emitted from a light source into light beams of different wavelength bands at different times and directs the light beams to the spatial light modulator without returning them to a light source and a means for processing and/or using unwanted light transmitted through the reflection type color wheel.
Since the image projector of the present invention comprised as described above uses a reflection type color wheel to direct the light beams to the spatial light modulator without returning them to the light source and pass the unwanted light, it becomes possible to prevent the color filters from being heated and destroyed by absorption of light like a transmission type color wheel. Further the means for processing and/or using the unwanted light transmitted through the transmission type color wheel enables improvement of the heat dissipation and a reduction in the power consumption.